A1C is higher among individuals with diabetes mellitus (DM) when periodontal disease is present, and therefore treatment of chronic periodontitis is hypothesized to reduce A1C levels among individuals with DM. A meta-analysis of clinical trials showed that periodontal treatment reduced A1C by 0.4% at three months follow-up, however, these results are not definitive because the studies were heterogeneous, they had small sample sizes, and treatment was not always randomized. Moreover, the long term impact of periodontal treatment on A1C is unknown. Consequently periodontal treatment is currently not a recommended strategy to control A1C in DM. The American Academy of Periodontology (AAP) guidelines for effective treatment and control of chronic periodontitis consists of initial treatment followed by periodontal maintenance visits every 3 to 6 months. To the best of our knowledge the long-term strategy to treat and control periodontitis based on AAP guidelines has not been evaluated in relation to glycemic control among individuals with DM. We propose to evaluate the long-term effect (over 2 years) of AAP recommended treatment for chronic periodontitis on A1C among individuals with DM using data from the Veterans Administration computerized database in a prospective study design. To overcome the limitation of time-dependent confounding (periodontal maintenance visit determines subsequent periodontal treatment) we propose to use Marginal Structural Models (MSM) developed by Robins. This approach has, to our knowledge, not been previously used to assess oral-systemic disease relations. These analyses will therefore enhance the application of modern statistical methods to oral health research. The results from these analyses will approach those from a randomized controlled trial evaluating overall effect of long term periodontal care on A1C among individuals with DM. Controlling hemoglobin A1C in T2DM has been shown to prevent complications, yet just 44% of all Americans with DM have A1C within the recommended levels. A 1% reduction in A1C is associated with a 21% reduction for any diabetes-related endpoint among individuals with T2DM. If treatment of chronic periodontitis even modestly reduces A1C in individuals with DM, this will have important implications for the management of individuals with DM and periodontitis.